The fabrication of a semiconductor device involves a plurality of discrete and complex processes. For many of these processes, the semiconductor device may be disposed within a chamber, which is maintained at vacuum conditions. There are various devices that may be used to create these vacuum conditions. For example, in some embodiments, a turbomolecular pump may be used. In certain embodiments, a cryopump may be employed.
A cryopump includes a front surface, through which molecules may enter. Internal to the cryopump may be surfaces cooled to extremely low temperature through the use of closed loop compressed helium or other cooled fluids. Molecules that contact the cryogenic surfaces within the cryopump lose their thermal energy, condense and change from a gaseous state to a solid state (i.e. frost or ice). This cryocondensation traps or captures the gas on the low temperature surfaces, effectively adhering to the cryogenic surface. This is the physical basis of the cryogenic pumping. In some embodiments, charcoal is used to adsorb the non-condensable molecules such as hydrogen and helium. This pumping action is known as cryotrapping.
The pumping speed S of a vacuum pump is the volumetric displacement rate or volumetric flow rate in units of L/sec. The latter definition is the value of the quotient Q/P anywhere in the vacuum system, where Q is the gas load in units of torr*L/sec and P is pressure in units of torr.
Eventually, the cryopump becomes saturated with condensate. This lowers the pumping speed and stops the cryopump from trapping any new molecules. Thus, once saturated, the condensate has to be removed. The process of removing the condensate from the cryopump is known as regeneration.
Regeneration may be performed by raising the temperature of the cryopump, so that the condensate evaporates. Once evaporated, these molecules are exhausted from the chamber. The cryopump is then cooled again, and the process repeats.
The regeneration process may cause the outgassing of the molecules that were previously captured within the cryopump. Therefore, in many embodiments, it is common to dispose the cryopump in a cavity disposed beyond the wall of the processing chamber. A movable gate is disposed in front of the cavity that contains the cryopump. When the cryopump is regenerated, the gate is closed, isolating the cavity from the processing chamber and allowing the evaporated molecules to be exhausted without affecting the processing chamber.
While this system is effective in ensuring that previously captured molecules are not released back into the processing chamber, this system has other drawbacks. For example, the front surface of the cryopump is typically set back from the processing chamber. The geometry of the volume that separates the front surface from the processing chamber may serve to limit the pumping speed, and thus, the efficiency of the cryopump.
To illustrate this concept, consider a cryopump that is in communication with a chamber via a conduit of diameter D (in cm) and length L (in cm) where L is much less than D. The conductance C in L/sec for N2 gas at 25° C. is expressed as:
  C  =      9.14    *                  D        2                    1        +                  3          *                      L                          4              *              D                                          The conductance C, when placed in series with a pump onto a chamber, has the effect of reducing the pump speed by:
      S    eff    =            S      *      C              S      +      C      where Seff is the effective pump speed.
In present systems, the loss of pump speed may be significant. A cryopump with a N2 gas pump speed of 1100 Liters per second on a conductance with a length of 12 cm and a diameter of 32 cm may reduce the pump speed by as much as 60 percent of the original pump speed. This has economic consequences in that more pumps may be added to the system to achieve the desired pump speed.
Therefore, it would be beneficial if there were a cryopump arrangement that was able to isolate the regeneration process from the processing chamber, but did not suffer from the pumping speed issues of the present systems.